In well completion operations, a casing or liner is suspended in the borehole, and cement is pumped down the casing. The cement flows back up the annulus between the outside of the casing and the wall of the borehole. In a remedial/squeeze operation, the cement slurry is pumped under pressure through holes or perforation into annular or zonal voids. One problem encountered in these operations is that filtrate from the cement can cause damage to the adjacent formation.
If the well has zones which contain clays or shales which are sensitive to fresh water, or more specifically to cement filtrate, these zones could be damaged during the cementing operation.
There are two main families of materials used to prevent damage to clay or shale zones; clay stabilizers and clay controllers. Clay stabilizers are usually organic barrier type materials which adsorb onto the clays and are consumed. In this way they prohibit swelling and/or migration and are generally considered to be permanent. These materials are usually very effective and are often used in stimulation treatments. Because they are consumed, care must be exercised to utilize sufficient volumes throughout an application. Due to the relative high cost of these materials, they are not capable of treating deep clays or shales and are not widely used in cement operations. Most cement methods use clay controlling agents such as salt (NaCl) and KCl. Clay controllers are effective only as long as the solutions containing the material are in contact with the clays. These materials are generally less expensive than permanent clay stabilizers and since they are not normally adsorbed, they are not consumed. The quaternary amine complex materials disclosed here are considered to be clay control materials.
In addition to the possibility of damaging formation permeability with cement filtrate, there is another area where untreated cement filtrate can cause significant problems. If the water in the cement slurry destabilizes sensitive sections of a hole, for example in shales, extreme cases can result in a complete failure of the primary cement placement. In these cases, remedial cementing must be performed. Many times, even if the remedial cementing is considered a success, it still does not deliver the zonal isolation and casing protection possible in a successful primary cement operation.
Given the serious implications, concerned operators and service companies have tried for years to combat both situations with varying degrees of success. Following the example of mud companies, various loadings of salt (sodium chloride) are often used in cement formulations to protect against shale and clay problems. The concentrations of salt can be between 5% and 37.2% (salt saturated) by weight of mix water (BWOW) and the effects on cement performance differ depending on the concentration used. Concentrations of less than 10% tend to accelerate set times, concentrations between 10% and 18% have little effect, and concentrations greater than 18% retard set times. In addition, lower concentrations of salt may act to disperse or thin cement slurries, while salt saturation can cause an increase in slurry viscosity and associated mixing difficulties. Fluid loss control agents can also be severely degraded by higher salt concentrations, often necessitating higher loadings of the control agent to maintain the desired level of fluid loss. High salt concentrations can also contribute to excessive air entrainment during mixing requiring chemical defoamers. While most of these side effects can usually be compensated for with careful testing and addition of other additives, this can be a demanding and expensive solution to clay control. Since the effect of salt depends directly on its concentration, unintentional variations in concentration, caused by loading errors, poor blending, or deblending during transit, can result in a cement system that does not perform well in the field.
In an effort to avoid the inherent problems associated with adding salt to cements, some operators turn to potassium chloride (KCl) to prevent clay problems. In many clays, the superiority of the potassium ion over sodium in terms of stabilization allows for much lower concentrations of KCl to give equal or even better results than higher loadings of salt. KCl loadings are usually between 2% and 5% BWOW. Loadings of KCl higher than 8% BWOW tend to cause extreme slurry gelation problems and are therefore not commonly used. The lower loadings of KCl are often more economical than salt and since the range of concentrations used is so much smaller, the variations in side effect across that range is very limited. Although the effects tend to vary in magnitude with curing temperature, like salt, low concentrations of KCl can act as an accelerator of cement set times and compressive strength development. In most slurries, KCl tends to be a more effective accelerator than an equal amount of NaCl. KCl also has been found to degrade the performance of at least some fluid loss control additives even more than an equivalent amount of salt. Unlike salt, it has been shown that even at the relatively low concentration of 5%, KCl can increase slurry viscosity. As with salt, additional additives such as fluid loss control, retarders, dispersants, etc. and testing are usually necessary to successfully utilize KCl as a cement clay control.
In using either salt or KCl, there are occasional situations where necessary slurry parameters have to be compromised or even sacrificed due to difficulties in overcoming certain adverse effects of these two materials. Additionally, it has been found that the apparent low cost of salt or KCl as cement clay control additives can be multiplied many times over when the cost of necessary remedial materials is considered.